Capacttor and circuit



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Patented Oct. 20, 1953 UNITED STATES OFFICE CAPACITOR AND CIRCUIT William H. Cherry, Princeton, N. J., assignor to Radio Corporation of America, a corporation of Delaware Claims.

This invention relates to capacitors, and more particularly to an electronically-variable capaciitor and a circuit utilizing the same.

An object of this invention is to provide an electronically-variable capacitor oi rapid response.

Another object is to devise an electronicallyvariable capacitor which provides a large range of capacitance variation.

Another object is to devise an electronicallyvariable capacitor by which the frequency of an electronic oscillator may be altered or controlled.

A further object is to provide a novel and effective automatic frequency-control circuit.

A still further object is to devise an automatic frequency-control circuit which is capable oi use for controlling the frequency of a local oscillator in television and frequency modulation receivers, as well as in amplitude modulation receivers.

The foregoing and other objects of the invention will be best understood from the following description of some exemplincations thereof, reference being had to the accompanying drawing, wherein:

Fig. 1 is a basic circuit useful in explaining the operation of this invention;

Fig. 2 is a modification of Fig. l;

Fig. 3 is an AFC circuit utilizing an arrangement somewhat similar to Fig. 2; and

Fig. 4 is a modification of a portion of Fig. 3.

The objects of this invention are accomplished, brieily, in the following manner:

At high frequencies, the electrostatic capacitance existing between the electrodes of a gaseous discharge tube, either of the cold cathode glow, hot cathode arc, or other gaseous discharge type, when no direct current is flowing therethrough is markedly changed by the presence of the discharge when direct current is flowing. By direct current is meant herein unidirectional current though variable even at audio frequency rates and faster. In glow tubes this direction may itself be reversed and the capacity variations under discussion may still be obtained. The value of this capacitance changes with changes in the value of the direct current fiowing through the glow tube itself. Speciiically, when the direct current flowing increases, the capacitance increases, and the capacitance decreases when such current decreases. By connecting the electrodes of a glow discharge tube in a direct current circuit the current flow in which circuit is controlled by a control voltage derived in turn from a suitable condition-responsive network, the capacitance between such electrodes is varied by such control voltage and this variable capacitance is in turn utilized to control the frequency of an oscillator. If the condition-responsive network produces a control voltage in dependence upon the amount of mistuning of the oscillator, the arrangement of this invention provides an AFC system for the oscillator.

At suiciently high frequencies, the plasma of a gas discharge does not behave as at low frequencies, as a conducting medium, but exhibits both capacitive and inductive effects, depending upon the frequency, these eects being presumably attributable to the relative motions of the space charge forming electrons and. the space charge neutralizing ions in the discharge. Therefore, at high frequencies, the electrostatic capacitance vexisting between the electrodes of a glow discharge tube when no direct current is flowing therethrough is markedly changed by the presence of the discharge when direct current is flowing. This eifect has been measured on an OB2 voltage regulator glow discharge tube and capacitance variations of more than two to one have been observed when the direct current is varied from minimum to rated maximum for the discharge tube. Similar measurements have been made on many other discharge tubes, notably the NEI? neonglow tube.

Moreover, by changes in the density of the particles constituting the main plasma of the glow discharge, in other words, of the positive ions and negative electrons, corresponding changes or variations in R. F. capacitance between the main electrodes of the glow discharge are produced. These changes in the density of the particles constituting the main plasma are produced by varying the entire glow discharge current. words, changes in the direct current of the glow discharge produce corresponding changes in the dielectric constant of the main plasma, and corresponding changes in the capacitance between the two maink electrodes of the glow discharge tube; when the glow discharge takes place in a conventional two-electrode glow tube, these changes in R. F. capacitance will take place between the two electrodes of the tube. Therefore, relatively inexpensive two-electrode glow tubes may be used for the purposes of this invention, tubes having auxiliary electrodes not being required. When the direct current flowing increases, the capacitance usually increases, Vand the capacitance usually decreases when such current decreases.

Now referring to Fig. l, which shows schematically a basic or elemental form of circuit for ln other f utilizing this invention, a gas discharge tube I, which may be a type CB2 voltage regulator or other suitable glow discharge tube, has a pair of electrodes 2 and 3 therein, electrode 3 being grounded. The interelectrode space of glow discharge tube I is connected in series in a capacitance-controlling direct current circuit which comprises -a terminal Il connected to the positive side of a suitable source of direct current, a variable resistor 5 which functions-as a current controlling element, an inductance 8 which has a high impedance at radio frequencies andisolates the direct current source from radio frequencies, electrodes 2 and 3, and ground Aor lthe negative side of the source. The direct vcurrent source has a potential which is appropriate to establish a glow discharge in tube I, and by varying resistor 5, the glow discharge current, or the direct current in the main plasma of the glow discharge, may be varied.

According to `this invention, as previously stated, this Variation of the glow discharge current pro duces a capacitance variation between the two electrodes 2 and 3. This capacitance variation, which depends on the direct current owing in tube I and which itself appears at high frequencies, may be utilized in a high frequency circuit by connecting a high'frequency terminal I to electrode -2 through a direct current isolation condenser 8 and by connecting another highfrequency terminal 9 to ground. Then, the high frequency ycapacitancevariation, as'resistor' is varied to vary the direct -current flow, appears between terminals I and "5.

Fig. 2 shows a'modication of Fig. 1 Vin'which the variation of capacitance with direct current changes is twice as great asin Fig. 1, "In Fig. 2,

' elements the same as those of Fig. `1 are denoted by the same reference numerals. In Fig. 2, ithe path for the flow of capacitance-controlling direct current is as follows: positive source terrninal 4, variable resistor 5, one electrode Ill of a first glow dischargetube II,'the opposite electrode I2 of'tube II, one electrode I3 of a'second glow discharge tube I4, and the opposite electrode I5 of tube M'togroundand the'negative side ofthe source. Thus, the direct current flows through two separate glow discharge paths in Fig. 2, and therefore is effective to Vary `two separate high frequency capacitances inresponse lto variation of resistor 5,v rather than only one "as in Fig. 1. No inductance, resistance or other high frequency separating elements, such as 6 in Fig. 1, are needed in this case.

Electrode Ill is grounded for highfrequencies by a suitable condenser I6. Therefore, the net or effective high frequency capacitance between terminal 1 and ground or terminal -9 '(electrode I5 being grounded directly as illustrated) consists of two variable capacitances in parallel, one being the plasma of tube II and the other the plasma of tube I 4. Both `of these capacitances are variable by the variation of direct current flow in the series direct current path through both tubes, and since capacitances in parallel add to give the total over all capacitance, the variation of the high frequency capacitance -between terminals 'I and 9 with variation in direct current is twice as great in Fig. 2 as in Fig. 1. A direct current isolation condenser 8 is connected between terminals A'I and the directly-connected electrodes I2 and I3.

Fig. 3 illustrates the application of the concept of this invention as disclosed in Fig. 2te an AFC circuit for automatically controlling the ire- 4 quency of a local oscillator in a television sound receiver or in a frequency modulation receiver. The path for the flow of capacitance-controlling direct current is as follows: positive terminal Il, electrode i3 of glow discharge tube II, electrode I2 of tube II, electrode I3 of glow discharge tube I4, electrode I5 of tube ill, anode I'I of a direct current amplifying triode I8, and cathode I8 of said triode to ground and the negative terminal of the direct current source. Since the anode- -cathode path of triode I8 is in series in the direct current circuit through the glow discharge tubes I I and I4, variation of the effective impedance offsaidpathby variation of the potential applied Ato the control grid of this triode varies the amount of direct current owing through theglowdischarge tubes and correspondingly varies the net high frequency capacitance provided by such glow tubes.

A frequency modulation discriminator or ratio detector, indicated at Y2li inthe form o Va. block because itis more or less conventional, constitutes a part of the frequency modulation receiver the local oscillator frequency of which is to lbe automatically controlled. In the output lead of such a discriminator or ratio detector there appears, as is well known, audio frequency amplitude variations representing intelligence, and in addition a direct voltage component appears in this lead whenever `the kfrequency modulation receiver is mistuned, that is, wheneverthe center or rest frequency of the heterodyned incoming signal is not exactly at the crossover point of the discrirninator characteristic because of mistuning of the local oscillator in the receiver, the magnitude and polarity of said component corresponding to the degree and relative direction 0f rnistuning of such oscillator. The Voutput lead of unit 253 is coupled to an audoamplier as indi cated in order to apply the audio frequencyamplitude variations appearing in such output lead thereto for utilization'therein. The direct 'voltage component appearing in the `outputlead :of unit 20 is applied, through an RC ne.tWorl. consisting Yof -a seriesresistor '2l and va shunt condenser 22 connected from the end of `resistor ZI opposite unit .2u to groundyto the control grid 23 of the triode I8. The RC network 2I-22 serves as an audio filter, `to lter out or bypass audio frequencies, or to prevent them from reaching the grid .23, so that the only voltage .eiective on such grid is the direct voltagein the output of unit 2li. Triode I8 is connected to act as a direct current amplier, to amplify the direct voltage from unit 20, the anode-cathode path of triode I8 being controlled by, or being responsive to, the direct voltage output of unit `2Q appliedv to control grid 23. Variations of the effective impedance of anode-cathode path I1, YI9 by varia tions of the potential applied to grid 23 produce corresponding variations of the amount of direct current flowing through the glow discharge tubes II, I 4, in turn producing corresponding variations of the net high frequency capacitance provided by such glow tubes.

Similarly to Fig. 2, the directly-connected electrodes I2 and I3 of glow tubes II and I4, respectively, are connected through a direct current isolation condenser 8 to a high frequency network 25, in this case consisting of the tank circuit 26 of a local oscillator in the receiver, one end of said tank circuit being grounded. As in Fig. 2, the variable high frequency capacitance, which is dependent on the amount of direct current flowing through the glow tubes I I and I4, appears or is provided between the directly-connected electrodes I2 and I3 and ground. Electrodes I2 and I3 are connected through condenser 8 to the upper end of oscillator tank circuit 26 and the lower end of said tank circuit is grounded, so that the variable high frequency capacitance of the glow tubes is connected across or in shunt with the oscillator tank circuit 25. In order to complete the high frequency circuits including the glow tube variable capacitance to ground, electrode III is grounded for high frequencies by condenser I 6, while electrode I5 is grounded for high frequencies by condenser 24.

In order to explain the operation of the AFC circuit of Fig. 3, let us assume that the frequency of the local oscillator has tended to drift to a higher frequency than is proper. This means that the center or rest frequency of the heterodyned incoming signal is not exactly at the crossover point of the characteristic of discriminator unit 2t, and unit 20 is connected to provide a positive direct voltage component in its output lead under these conditions. This positive potential passes through the audio frequency filter 2|, 22 and is applied to control grid 23, reducing the effective impedance of the anode-cathode path of triode I8 in an amplified fashion. 'Ihis`V reduction of effective impedance of path I1, I 9 reduces the total impedance of the direct current circuit including such path, increasing the flow of direct current through the glow tubes II, I4 and increasing the high frequency capacitance between the directly-connected electrodes I2, I3 and ground. As in Fig. 2', the two glow tubes have their high frequency capacitances in shunt as regards the oscillator tank circuit 2E, while their direct current paths are in series. An increase in the capacitance provided by the glow tubes, which capacitance is in shunt to the oscillator tank 26, causes a decrease in the frequency of the oscillator controlled by such tank circuit, this decrease counteracting the initiallyassumed increase in local oscillator frequency and effecting automatic frequency control of such oscillator.

Conversely, when the local oscillator frequency tends to decrease, a negative direct voltage is produced at the output of unit 20, decreasing the flow of direct current through the glow tubes, thereby decreasing the high frequency capacitance provided by them and increasing the frequency of the oscillator controlled by tank circuit 26. g

The response of the AFC circuit of this invention, containing as it does only electronic components, is extremely rapid. At the same time, it has been found that the effective frequency control range of the AFC circuit of Fig. 3 is quite large, being on the order of 1.5 megacycles, for example. With the feedback loop of Fig. 3, in fact, it is possible to so alter the reception frequency of the receiver, by alteration of the frequency of the local oscillator, as to maintain the reception of a carrier which is at first only approximately tuned in.

The arrangement of this invention'is substantially independent of warm--up time of the receiver and of line voltage uctuations and of changes in the B supply voltage of the receiver. It can be installed in the field if necessaryand requires no unusual or special equipment or no new tube structures.

Due to the fact that, according to this invention, the variation of high frequency capacitance between the two principal electrodes of the glow discharge by the change in ion and electron density in the plasma between them, the plasma being regarded as a dielectric material, is of the order of several hundred per cent of the static capacitance, sweep searching of an oscillator frequency may be readily effected merely by applying appropriate sweeping direct voltages to the glow discharge device to vary the high frequency capacitance provided thereby through an appropriate range, the glow discharge device being connected to control the oscillator frequency in the manner disclosed previously. Thus, sweeping in an AFC circuit of a receiver may be easily effected. Again, this invention may be readily applied to provide sweep generators, such as those used for aligning filters and intermediate frequency amplifiers, replacing the cumbersome and expensive mechanical contrivances used heretofore to accomplish such sweeping. Also, frequency modulation of an oscillator may be carried out by applying appropriate modulating voltages to the glow discharge device to vary the high frequency capacitance provided thereby.

It is not absolutely necessary to provide an extra or additional tube, such as tube I8 of Fig. 3, for amplification of the direct voltage output of the discriminator unit 20, which voltage is used as a capacitance-controlling voltage for the glow tubes II and I4. Fig. 4 is a diagrammatic representation of a modified circuit in which an additional tube is not required. Tube 21 is a pentode and constitutes an I. F. amplifying stage having its control grid 28 supplied from the secondary of an I. F. transformer 29 which is tuned by a condenser 3B. yScreen grid 3| of pentode 21 is connected to a suitable positive potential source as shown, the usual dropping resistor and bypass condenser being provided as indicated. The anode 32 of tube 21 is coupled to supply amplified I. F. energy to the tuned primary 33 of an I. F, output transformer the secondary 35 of which is also tuned. The amplified I. F.V output of tube 21 appears in secondary 35 and is applied to succeeding stages.

Anode 32 is supplied with potential from positive terminal 4 through a pair of series-connected glow tubes (not shown) such as II and I4 of Fig. 3 and a resistor 36 bypassed by a condenser 31. Thus, the direct current owing in the anodecathode path of tube 21 flows also in the gaseous discharge paths of the two glow tubes, and variations of direct current flow in tube 21 result in corresponding variations of the capacitance between the electrodes of the glow tubes. At the same time, tube 21 amplifies the I. F. signals in the conventional manner.

In order to vary the D. C'. flow in tube 21, to

thereby vary the D. C. ow through the glow tubes and thereby also their' effective capacitance, or in order to amplify the D. C. output of the discriminator unit 20 before applying it to the glow tubes, a connection extends from the output of the discriminator, by way of a series resistor 38 and a shunt condenser 39,v to the secondary of I. F. input transformer 29 and thereby also to grid 28. Direct voltages appearing in the discriminator output are thus applied to grid 28 to vary the grid bias of tube 21 and thereby also to vary the D. C. iiowing through such tube and also through the glow tubes. Such direct voltages are subjected to amplification in the process.

The use of tube 21 as a D. C. amplifier for the AFC circuit does not interfere in any appreciable Way with its use as an I. F. amplifier, said .tube functioning in a more or less conventional way to amplify I. signals applied theretoI from input transformer 29 and to apply such. signals to output transformer 34. Therefore, by utilizing also as a D. C. amplifier forthe AFC circuit an l'.- F. ampli-fier tube 27 already and normally present in the receiver, an extra or additional tube is not needed in Fig. 4 for D. C. amplification for the AFC arrangement.

What I claim to be my invention is as follows:

1. A variable high frequency capacitance circuit, comprising a gaseous discharge device hav- .ing only two electrodes between which a glow discharge may occur, means for causing a flow of substantially unidirectional current between said two electrodes and through the gaseous medium to ionize said medium and establish a gaseous discharge in said. device, means for varying the amount of current nowing between said two electrodes to vary the high frequency capacitance between said two electrodes, and means for connecting said two electrodes to a high. frequency circuit for utilization of said highfrequency capacitance.

2r A variable high frequency capacitance circuit, comprising a gaseous discharge device having only two electrodes between which a glow discharge may occur, means for connecting said two electrodes through a direct current blocking condenser to a high frequency circuit for utilization of the highV frequency capacitance therebetween, a source of unidirectional current, means connecting one of said two electrodes to one terminal of said source and the other of said two electrodes through a high frequency choke to the other terminal of said source, whereby a ow of unidirectional current may be produced between said two electrodes and through the gaseous medium to ionize said medium and establish a gaseousy discharge in said device, and means for varying the amount of current flowing between said two electrodes to vary said capacitance.

3. A variable high frequency capacitance circuit, comprising a pair of gaseous discharge devices each having only two electrodes between which a glow discharge may occur, a source of direct current, means connecting said two devices in series across said source, said source having a voltage such as to cause a now of direct current through the series circuit including the said electrodes of said two devices to establish gaseous discharges in said devices, means for connecting the two directly-connected. electrodes of the two devices through a direct current blocking condenser toa high frequency circuit, means grounding for high. frequenciesthe other electrode of each device, said second-named means and said last-named means providing for utilization of the high frequency capacitance between said two directly-connected electrodes and ground, and means in circuit with said source and said twodevices for varying the amount of direct current owing through said series circuit including said electrodes to vary said capacitance.

4. A variable high frequency capacitance cir'- cuit, comprising a gaseous glow discharge device having a pair of electrodes between which va glow discharge may occur, means for causing a flow of substantially unidirectional current' between said pair of electrodes' andy through the gaseous medium to ionize said medium and establish a glow discharge in said device, means responsive to the tuning of a high frequency oscillatorA for producing a control voltage proportional to the amount `of mistuning of said oscillator, meansresponsive to said control. voltage for varying the amount of current flowing through said. device to vary the high frequency capacitance between said pair of electrodes, and means connecting said pair of electrodes to the frequency-controlling' circuit of said oscillator to vary the output frequency of the same, by means of the variations in said capacitance, to alter the tuning ofsaid oscillator to the proper output frequency.

5. A variable high frequency capacitance circuit, comprising a pair of gaseous discharge devices each having a pair of electrodes between which a discharge may occur, a source of direct current, means connecting said two devices in series across said source, said source having a voltage such as to cause a flow of direct current through the series circuit including said two devices to establish gaseous discharges in said devices, means for producing a control voltage, and means in circuit with said source and said two devices, said last-named means being responsive to said control voltage for varying the amount of direct current owing through said series circuit to vary the high frequency capacitance between the two directly-connected electrodes of the two devices and one of the other electrodes.

6. A variable high frequency capacitance circuit, comprising a pair of gaseous glow discharge devices each having a pair of. electrodes between which a glow discharge may occur, a source of substantially unidirectional current, means connecting said two devices in series across said source, said source having a voltage such as to cause a flow of current through the series circuit including said two devices to establish glow dischargesy in said devices, means coupling the two directly-connected electrodes of the two devices and one ofthe other electrodes to a high frequency circuit for utilization of the high frequency capacitance between the electrodes so coupled to such high frequency circuit, means for producing a control voltage, and means in circuit with said source and said two devices, said last-named means being responsive to said control voltage for varying the amount' of current flowing through said series circuit to vary said capacitance and thereby to control said high frequency circuit. y 7 A circuit as defined in claim 6, wherein the last-named means is connected in series with the source and the two devices and wherein said lastnamed means comprises the output electrode current path of an electron control device to the control electrode of which the control voltage is applied.

8. A variable high frequency capacitance circuit, comprising a pair of gaseous discharge devices each having a pair of electrodes between which a discharge may occur, a source of substantially unidirectional current, means connecting said two devices in series across said source, said source having a voltage such as to cause a iiow of current through the series circuit including said two devices to establish gaseous discharges in said devices, means coupling the two directly-connected electrodes of the two devices and one of the other electrodes to the frequencycontrolling circuit of a high frequency oscillator for utilization of the high frequency capacitance between the electrodes so coupled to said lastnamed circuit, means responsive to the tuning of said oscillator for producing a control voltage proportional. to the amount of misto-ning of said oscillator, and .means in circuitv with saidv source and said two devices, said last-named means being responsive to said control voltage for varying the amount of current flowing through said series circuit to vary said capacitance and thereby to vary the tuning of said oscillator to counteract its mistuning.

9. A circuit as dened in claim 8, wherein the last-named means is connected in series with the source and the two devices and wherein said last-named means comprises the output electrode current path of an electron control device to the control electrode of which the control voltage is applied.

10. A variable high frequency capacitance circuit, comprising a pair of gaseous glow discharge devices each having a pair of electrodes between 15 which a glow discharge may occur, a source of direct current, means connecting said two devices in series across said source, said source having a voltage such as to cause a ilow of direct current through the series circuit including said two devices to establish glow discharges in said devices, means coupling the two directly-connected electrodes of the two devices and one of the other electrodes to the frequency-controlling circuit of a high frequency heterodyning oscillator for g5 utilization of the high frequency capacitance be-V tween the electrodes so coupled to said lastnamed circuit, discriminator means responsive to the tuning of said oscillator for producing a direct control voltage proportional to the amount of mistuning of said oscillator, and means in circuit with said source and with said two devices, said last-named means being responsive to said control voltage for varying the amount of direct current flowing through said series circuit to vary said capacitance and thereby to vary the tuning of said oscillator to counteract its mistuning.

WILLIAM H. CHERRY.

References Cited in the ille of this patent UNITED STATES PATENTS VNumber Name Date 2,012,710 Crosby Aug. 27, 1935 2,032,620 Langmuir Mar. 3, 1936 2,033,231 Crosby Mar. 10, 1936 2,077,223 Crosby i Apr. 13, 1937 2,407,424 Hollingsworth Sept. 10, 1946 

